Protection of metallic objects by galvanic action



Patented July 29, 1952 PROTECTION OF METALLIC OBJECTS BY GALVANIC ACTION Herbert Manfred Freud, dit Jean Frasch,

Nantcrre, France No Drawing. Application July 12, 1946, Serial No. 683,152. In France August 7, 1945 6 Claims.

1 In the U. S. patent application Serial No 314,349 filed January 17, 1940, on which U. S Patent No. 2,463,483 issued March 1, 1949, and

corresponding to French Patent 855,683, I have described a process of obtaining a corrosionresistant protective layer on magnesium or alloys rich in magnesium, to prevent corrosion of the latter. This is accomplished by galvanic action in a bath containing ions of manganese, also containing bichromate ions and optionally small quantities of other ions, under the conditions that the solutions thus constituted do not, of themselves, chemically attack magnesium.

In this application, the terms alloys rich in -magnesiumand magnesium-rich alloys are intended to cover alloys consisting of magnesium together with small percentages of other metals, e. g. manganese, zinc or aluminum, or two or more of such metals. Such alloys, as in the prior art, usually contain well over 85% of Mg. The term magnesium-rich metal is intended to cover such alloys as well as magnesium metal itself.

In continuing my researches I have established that the manganese cation does not constitute an isolated case in this type of treatment and it can be replaced, partly or wholly by other cations such as those whose bichromates are soluble in water and of which the hydroxides are insoluble in water, notably chromium, aluminum, and zinc. The principal anion with which these cations are associated is still the bichromate ion.

The galvanic treatment of magnesium in these solutions (magnesium being the anode and another more noble conductor such as carbon being the cathode) leads to the formation on the surface of the magnesium, of a layer composed essentially of an oxide of chromium of a degree of oxidation lower than CrOa associated with a very small quantity of an oxide of the cation metal found in the solution. Since the bath (electrolyte) is without any chemical action on metallic magnesium, it will be understood that the coating on the magnesium article, after washing and drying, does not contain any magnesium compounds. This treatment is effected by the galvanic action of the cell itself, without supplying electric current from outside.

All the bichromates of the cation metals, soluble in water and having a pH above 2, and the hydroxides of which cation metals are insoluble in water (or not more than very slightly soluble in water) can, in principle, be used. Thus the use of alkaline or alkaline earth cations are excluded, as Well as cations which form insoluble rest Mg. In open circuit the E. M. F. is 1.7 volts.

2 bichromates, notably Ba++, Ag Hg++, Pb++', etc.

The pH of the solutions (galvanic baths) should be above 2, and preferably between 2.5 and 3.5.

The solution can contain, in addition to the cation of the kind described, combined with the anion HCrO4 or Cr2O7--, small quantities of such anions as SO4-, in a proportion lower than 0.1 a necessary condition is that the solution does not chemically attack magnesium (metal). However, the bath must be free from halides which would produce a chemical attack even when present only in small amounts.

These solutions can be prepared for example by the action of chromic acid on a carbonate such as ZnCOa, or by action of chromic acid on a freshly precipitated hydroxide such as AKOH) a I or by reduction of a part of the chromic acid with formation of bichromate of chromium. In a similar manner the cerium bichromate solution can be made by dissolving the common cerium carbonate (which is the cerous compound) in aqueous chromatic acid solution.

It is obvious that in making the bath, the proportions of the materials used should be such as to give solutions having a pH above 2 and preferably between 2.5 and 3.5.

The galvanic treatment properly so called, is carried out in conformity with the principal patent, i. e. by immersing in the galvanic bath, on the one hand a magnesium object or magnesium alloy object which forms the anode and on the other hand a more noble electrode, for example carbon which forms the cathode, and closing theexternal circuit over a reg-ulable resistance. The duration of the treatment is usually half an hour to two hours, depending on the shape of the piece to be treated and on the thicknesss of the protective layer which it is desired to form on the surface of the article.

The following examples are given for further explaining the invention.

Example 1 A solution is prepared by dissolving 40 gr. of CIO3 in a liter of water. 23 grams of zinc carbonate are added and allowed to dissolve therein. The pH is 2.9. x

A plate of Mg-alloy, having a total surface area (on both sides) of 1 square decimeter is placed in the solution as anode. A carbon cathode of about the same size is also placed in the solution. In this case the Mg alloy may contain 1.8% Mn The circuit is then closed by connecting the-Mg- Mn alloy and the carbon pieces outside of the cell, the resistance adjusted to give an initial current of 60 milliamperes under a tension of 0.85 volt. After allowing the current to flow for 4 hour, the Mg-alloy piece is taken out of the bath, and is found to have a deep brown coating. It is then washed and dried.

Example 2 13.2 grams of aluminium sulphate,

A12(SO4) 3.18H2O nesium containing 6% aluminum, 1% zinc and 0.4% manganese, in the form of a plate 1 decime- Example 3 60 grams of CrO; are dissolved in a liter of water,'then 60 grams of oxalic acid (COOH) 2.21120 are added, the mixture allowed to react cold, requiring about 3 hours. The oxalic acid reduces a portion of the CrOa, giving bichromate of chromium (still containing an excess of CrOa). Then 27.7 grams MnCOs are added. After allowing to react for an hour, the solution is filtered. The filtrate has a pH of 3.35.

This gives a solution which contains 42.5

gr. CrzOv, 9.4 grams of Mn++ and 6.2 grams of Cr+++ per liter and which solution does not chemically attack magnesium metal.

A plate, having a total surface area (on both sides) of 1 square decimeter of Mg alloy containing 4% Al, 3% Zn and 0.4% Mn is immersed in the bath as anode, and a carbon plate of about the same size constitutes the cathode In open circuit the voltage is 1.85. The circuit (including an adjustable resistance) is closed, giving an initial current of 75 milliamperes at 0.85 volt.

The current is allowed to flow for an hour, the anode removed, washed and dried. It is found to be covered with a deep gray coating.

Example 4 40 gramsof CrOa are dissolved in a liter of water, with 13 grams of MnCOs and 11 grams of ZnCOs. The pH is 3.1.

A plate of Mg alloy, having a total surface area (on both sides) of 1 square decimeter is placed in the solution as anode. A steel cathode of the same size is also placed in the solution. In open circuit the E. M. F. is 1.4 volts. The circuit is then closed by connecting the Mg alloy and the steel pieces outside of the cell, the resistance is adjusted to give an initial current of 50 milliamperes under a voltage of 0.7 volt.

After allowing the current to flow for 1 hour, the Mg alloy piece is taken out of the bath, and is found to have a deep gray coating.

The above examples are given in an illustrative and not limitive sense. The nature of the cation can be varied, so long as the bichromate formed is soluble in water and the corresponding hydroxide is insoluble in water or substantially so.

Coatings are obtained, which always contain an oxide of chromium of which exact chemical composition has not been determined with accuracy. I-Iowever the chromium in the coating is of a stage of oxidation between CrOa and CrzOa. The said oxide is in amorphous state. It can be accompanied by the oxide of the cation present in the electrolyte. These coatings afford good protection against corrosion of the Mg or Mg-alloy article.

The protective action of the coating can be improved, as specified in the main patent application, by treatment in a bath of molten paraffin at about to 200 C., or by other heavy hydrocarbon applied hot.

In the examples carbon was given as the preferred cathode, but it will be understood that other cathodes namely such as substances more noble than magnesium or magnesium alloy can be employed.

Thus steel can be used for constituting the said cathodes. However, carbon seems practically preferable because less polarization takes place and because the depolarization of carbon electrodes is easily performed by means of nitric acid.

I claim:

1. A process of protecting a metallic object formed of magnesium-rich metal, which process comprises galvanically coating such an object, while serving as the anode in a galvanic cell in which the electrolyte consists essentially of an aqueous solution of a water soluble chromic acid salt of ametal selected from the group consisting of chromium, aluminum, and zinc, such process being conducted without applying any electric current from an outside source to said galvanic cell, and which solution is incapable of chemical action on magnesium metal, and which solution has a pH between about 2.5 and about 3.5.

2. A process of producing a protective film on a metallic object composed of a magnesium-rich metal which comprises galvanically coating such an object by causin such object to serve as the anode in a galvanic cell while said article is immersed in an electrolyte which consists essentially of an aqueous solution of a chromic acid salt of a metal selected from the group consisting of chromium, aluminum, and zinc, together with a dissolved chromic acid salt of manganese, which solution contains not more than 0.1% of S04 radical and is free from halides, and is incapable of chemically actingupon magnesium metal and has a pH of between about 2.5 and about 3.5.

3. A process of producing a protective film on a metallic object composed of a magnesium-rich metal which comprises galvanically coating such an object by causing such object to serve as the anode in a galvanic cell while said. article is immersed in an electrolyte which consists essentially of an aqueous solution of a chromic acid salt of a metal selected from the group consisting of chromium, aluminum and zinc, which solution contains some S04 ion, but substantially less than 0.1% thereof, and is devoid of halogen anion, and is incapable of chemically acting upon magnesium metal and has a pH of between about 2.5 and about 3.5.

4. A process as in claim 1 in which the article to be protected is formed of an alloy containing substantially over 85% of magnesium.

5. An article composed of magnesium-rich metal having a firmly adherent surface layer consisting essentially of an insoluble amorphous oxide of chromium, which oxide is less highly oxidized than CIOs, combined with a small amount of an oxide of a metal selected from the group 15 consisting of chromium, aluminum and zinc, such layer containing no magnesium compound.

6. An article as in claim 5 in which the coating also contains an insoluble compound of manganese.

HERBERT MANFRED FREUD,

DIT JEAN FRASCH.

REFERENCES CITED Ihe following references are of record in the file of this patent:

UNITED STATES PATENTS I Number Name Date 2,203,670 Buzzard June 11, 1940 2,206,028 Buzzard July 2, 1940 2,224,528 Sutton et a1 Dec. 10, 1940 2,356,575 Frasch Aug. 22, 1944 OTHER REFERENCES 

1. A PROCESS OF PROTECTING A METALLIC OBJECT FORMED OF MAGNESIUM-RICH METAL, WHICH PROCESS COMPRISES GALVANICALLY COATING SUCH AN OBJECT, WHILE SERVING AS THE ANODE IN A GALVANIC CELL IN WHICH THE ELECTROLYTE CONSISTS ESSENTIALLY OF AN AQUEOUS SOLUTION OF A WATER SOLUBLE CHROMIC ACID SALT OF A METAL SELECTED FROM THE GROUP CONSISTING OF CHROMIUM, ALUMINUM, AND ZINC, SUCH PROCESS BEING CONDUCTED WITHOUT APPLYING ANY ELECTRIC CURRENT FROM AN OUTSIDE SOURCE TO SAID GALVANIC CELL, AND WHICH SOLUTION IS INCAPABLE OF CHEMICAL ACTION ON MAGNESIUM METAL, AND WHICH SOLUTION HAS A PH BETWEEN ABOUT 2.5 AND ABOUT 3.5. 